How is billing done in case of a GPRS connection?

In GPRS, a customer pays to the service provider not for the number of hours the subscriber is attached to the GPRS network, but for the amount of data transferred. Since it is difficult to calculate the amount of data packets transferred, different network operators actually implement different charging techniques. For example, some network operators charge fixed monthly/yearly amount for the usage of network service.
There is no AT command that will display the billing amount for a GPRS connection.